Digital Audio Broadcasting (DAB) is a medium for providing digital quality audio, superior to existing analog broadcasting formats. FM In-Band On-Channel (IBOC) DAB can be transmitted in a hybrid format where the digitally modulated signal coexists with the currently broadcast analog FM signal, or in an all-digital format where the analog FM signal has been eliminated. IBOC requires no new spectral allocations because each DAB signal is transmitted within the spectral mask of an existing FM channel allocation. IBOC promotes economy of spectrum while enabling broadcasters to supply digital quality audio to their present base of listeners.
FM DAB systems have been the subject of several United States patents including Pat. Nos. 6,108,810; 5,949,796; 5,465,396; 5,315,583; 5,278,844 and 5,278,826. In an FM compatible digital audio broadcasting system, digitally encoded audio information is transmitted simultaneously with the existing analog FM signal channel. The advantages of digital transmission for audio include better signal quality with less noise and wider dynamic range than with existing FM radio channels. Initially the hybrid format would be used allowing existing receivers to continue to receive the analog FM signal while allowing new IBOC DAB receivers to also decode the digital signal. Some time in the future, when IBOC DAB receivers are abundant, broadcasters may elect to transmit the all-digital format. Hybrid IBOC DAB can provide virtual CD-quality stereo digital audio (plus data) while simultaneously transmitting the existing FM signal. All-digital IBOC DAB can provide virtual CD-quality stereo or multichannel audio along with an increased data channel.
One proposed FM IBOC DAB signal places orthogonal frequency division multiplexed (OFDM) subcarriers in the region from about 129 kHz to 199 kHz away from the FM center frequency, both above and below the spectrum occupied by an analog modulated host FM carrier. Some IBOC options permit subcarriers starting as close as 100 kHz away from the center frequency. The bandwidth of the existing analog FM signal is significantly smaller than the bandwidth occupied by the OFDM subcarriers. One concern involves how to allow the larger bandwidth OFDM signal into the radio while still maintaining the existing analog performance.
U.S. Pat. No. 6,178,317, which is hereby incorporated by reference, discloses a system for mitigating intermittent interruptions in a digital audio broadcasting system. The system includes a receiver for receiving a primary radio signal and a delayed redundant radio signal, demodulating the primary radio signal to provide the audio signal to a first output, and demodulating the delayed redundant radio signal to provide a second output. The receiver further includes a circuit for detecting degradation of at least one of the received signal components and a blending circuit for combining the primary audio signal and the redundant audio signal to form a composite audio signal. Additionally, the receiver includes an audio output circuit coupled to the blending circuit for converting the composite audio signal to an audible signal.
There is a need for an IBOC DAB receiver that can process an IBOC DAB signal in a manner that permits the blending and/or selection of the analog and/or the digital signal, without adversely affecting the analog signal path, for example so that the receiver can receive analog signals from broadcasters that have not yet switched to digital audio broadcasting without any performance impairment. One approach would be to use two switchable radio frequency filters in the radio tuner. One wideband filter (500 kHz) would be used for the digitally modulated OFDM signal and another narrowband filter would be used for the analog (180 kHz) signal. However, switching between the two filters can degrade the analog path. Such degradation in the analog path is not acceptable.
Alternatively, the larger bandwidth digitally modulated signal could be used until the blend to analog command is received. At this time a narrowband filter would be switched in. This could improve the analog performance but would filter out most of the digitally modulated signal. The main problem here is that with the narrowband filter in place, the digitally modulated signal is so limited that there is no way to determine when the digitally modulated signal is good enough to transition back to digital mode.
U.S. Pat. No. 6,178,317 discloses a digital audio broadcasting receiver having a front end that delivers intermediate frequency signals to each of two demodulators. However, U.S. Pat. No. 6,178,317 does not disclose any details of how the intermediate frequency signals are obtained.
This invention seeks to provide a method and apparatus for receiving an IBOC DAB signal, but does not impact the analog performance.